Altered thyroid states may influence the effects of psychotropic drugs. Prange (1963a) reported a case in which moderate factitious hyperthyroidism and Imipramine treatment seemed to interact to produce paroxysmal auricular tachycardia. Later, this author and his colleagues described increased lethality in imipramine in hyperthyroid mice (Prange et al., 1962) and decreased lethality in hypothyroid mice (Prange et al, 1963b). Similar effects have been found for phenothiazines (Ashford et al., 1968). More recently it has been shown that a variety of physiological and behavioural effects of phenothiazines are amplified in hyperthyroid mice (Park et al., 1973). Furthermore, Selye and Szabo (1972) found that thyroxine sensitized the rat to the toxic effects of haloperidol.